1. Field of the Invention
This invention relates to a method and apparatus for fixing heat-fusible toner images formed on a web by bringing the images into contact with a heated rotatable fixing member.
2. Description of the Prior Art
For example, in the field of electrophotography, an electrostatic latent image formed on a photosensitive medium is developed with toner particles electrically charged to the opposite polarity from that of the latent image. The toner image so formed is intactly fixed on the photosensitive medium or alternatively, it is first transferred from the photosensitive medium onto paper or synthetic resin sheet and then fixed thereon. For such fixation, heat is usually used. Generally, toner consists chiefly of heat-fusible synthetic resin which may be fused by heat and fixed on a toner image bearing member.
In general, there are two methods for fixing a toner image on an image bearing member by heat. One of the methods comprises applying radiant heat both to the toner image and the image bearing member, and the other method comprises bringing the toner image and the image bearing member into direct contact with a heated fixing member. The former method is low in efficiency of heat utilization and accordingly requires a heat source of great output, which means low safety and bulkiness of the apparatus. Recently, therefore, the latter method has become dominant which is high both in efficiency of heat utilization and safety and which leads to a smaller size of the apparatus. Examples of this latter method are disclosed in U.S. Pat. No. 3,452,181, U.S. Pat. No. 3,539,161, U.S. Pat. No. 3,945,726, etc. Any of these methods comprises passing a toner image bearing member through the nip formed between a pair of rollers. At least one of the rollers, namely, that roller which is in contact with the toner image bearing surface of the image bearing member, is heated by a heat source provided outside or within the roller. The methods of fixation disclosed in these patents are suited for fixing toner images formed on short cut bearing members or cut sheets. More particularly, these methods are suited for treating a multiplicity of cut sheets bearing toner images thereon continuously or at intervals, whereas they are not suitable for intermittently fixing toner images formed on a long footage of web.
U.S. Pat. No. 3,268,351 and U.S. Pat. No. 3,667,742 disclose methods of fixing toner images formed on a long footage of web. The methods of these patents comprises bringing a heated roller or a heated endless belt into contact with the toner image bearing surface of the web. The roller or the belt is rotated as the web is fed. Again, these methods are not suitable for intermittently treating the long footage of web. The reason is that, as is also the case with the aforementioned method of fixation for cut sheets, a pair of rollers or endless belts are urged against the image bearing member whenever the image bearing member is present between the pair of rollers or endless belts. Therefore, when fixation is interrupted, that portion of the web which is contacted by the heated fixing member may be degenerated or scorched or burnt. Further, if a toner image was present on said portion, that image would be destroyed by the idle rotation of the fixing member which is in contact with the web or, if such rotation of the fixing member was stopped to avoid this, that image might be too much fused and hot-offset to the fixing member. To avoid these inconveniences, rotation of the fixing member may be stopped and the temperature thereof may be lowered when the feeding of the web is stopped or when the fixation is intermitted, but the temperature once lowered would require much time to recover and this would offer an obstacle when the fixation is to be resumed.
U.S. Pat. No. 3,349,702 discloses another method of fixing toner images on a long footage of web. According to this method, a roller divided into a hot portion and a cold portion is normally brought into contact with the web and during movement of the web, the roller is rotated so as to bring the hot portion thereof into contact with the web and when the web is stopped, the roller is rotated so as to bring the cold portion thereof into contact with the web. This method can avoid the above-mentioned scorching or burning of the web, but the roller being brought into contact with the non-image bearing surface of the web results in not so good efficiency of heat utilization. This is because heat transfers only through the web to the toner to be fused. This in turn leads to the requirement of a high output for the heat source, which might also degenerate the web. This drawback would become more serious as the web is thicker. To avoid such a disadvantage, it would occur to mind to bring the roller divided into the hot and the cold portion into contact with the toner image bearing surface of the web, but this would be impossible in practice, since it would result in entirely unfixed toner images. In order for the toner images not to be disturbed, the roller during movement of the web must needs be rotated at the same velocity as the linear velocity of the web, whereas this would in turn bring the hot and the cold portion of the roller alternately into contact with the toner bearing surface of the web.